In the field of equipments for medical applications, imaging techniques are well-established for analyzing a body-part of a patient in a substantially non-invasive manner; for example, the imaging may be based on the recording of an echo signal that results from the application of ultrasound waves to the body-part. For this purpose, a contrast agent (for example, consisting of a suspension of phospholipid-stabilized gas-filled microbubbles in ultrasound applications) is typically administered to the patient; the contrast agent acts as an efficient (ultrasound) reflector, so that it enhances the visualization of blood in a vascular system within the body-part where it is present. Particularly, this technique is commonly exploited for the assessment of blood perfusion; indeed, as the contrast agent flows at the same velocity as the blood in the patient, its tracking provides information about the perfusion of the blood in the body-part under analysis.
Typically, the flow of the contrast agent is monitored by imaging the body-part during the perfusion process. More in detail, each image is defined by a matrix of pixel values indicative of the amplitude of the echo signal originating from corresponding portions of the body-part. For this purpose, the echo signal is usually compressed so as to adjust its amplitude to the smaller dynamic range that is commonly supported by video monitors. In order to obtain images with well-balanced contrast, the process always involves a non-linear compression; the operation is commonly based on a transfer function of the logarithmic type (and it is then referred to as log-compression).
Generally, in order to facilitate the tracking of the contrast agent, a contribution of any tissues of the body-part is at first reduced in the echo signal. This result may be achieved by acquiring the echo signal in a contrast-specific imaging mode. One example of such contrast-specific imaging is achieved by pulse inversion techniques. Other examples of contrast-specific imaging are achieved by power modulation techniques or by combinations of pulse inversion and power modulation techniques. Yet another example of contrast-specific imaging is disclosed in XP000764798 Arditi M. et al. “Preliminary Study in Differential Contrast Echography” Ultrasound in Medicine and Biology, Vol. 23, No. 8, pp. 1185-1194 1997, Elsevier, which is incorporated by reference. For this purpose, the cited document proposes processing the echo signal in two channels, whose signals are then subtracted; the processed signal so obtained is typically displayed in a linear gray-scale (i.e., with gray-levels proportional to the amplitude of the processed signal). In a specific implementation, the same processed signal is superimposed over an (unprocessed) log-compressed image.
In any case, the amplitude of either the video signal (i.e., the one obtained by compressing the amplitude of the echo signal) or of the contrast signal (i.e., the raw echo signal obtained as described in the cited document or by any other known contrast-specific imaging technique) is not in direct proportion to the local concentration of the contrast agent. As a matter of fact, only the power of the echo signal (i.e., the echo-power signal) exhibits a direct proportionality with the local concentration of the contrast agent.
With reference in particular to the video signal, as small differences in the echo signal (for example, with a ratio of 1.7 to 2.5, i.e., 20·log10(1.7)=5 dB to 20·log10(2.5)=8 dB) may be totally masked in the resulting compressed images, blood flow distributions with subtle variations or opacification heterogeneities (due to perfusion deficits) may thus be difficult to identify and can be easily overlooked. This hinders the detection of perfusion abnormalities, typically indicative of pathological conditions.
In any case, the resulting images strongly depend on the specific log-compression that is implemented by each type of equipment. Moreover, this process introduces subjectivity due to the setting of the log-compression according to different operator preferences. Therefore, the results obtained cannot be compared among operators using different equipments or settings.
On the other hand, a quantitative assessment of the perfusion process is provided by parametric analysis techniques. In this case, the video signal is preferably linearized so as to make its amplitude directly proportional to the local concentration of the contrast agent in the corresponding portions of the body-part. For this purpose, an inverse log-compression function is applied to the video signal, and the result so obtained is squared (so as to provide a signal in direct proportion to the local power of the original echo signal). The change over time of the linearized signal for each single pixel (or group of adjacent pixels) is fitted by a mathematical function. The mathematical function may then be used to calculate different perfusion parameters, which are indicative of corresponding haemodynamic and morphological characteristics of the corresponding portion of the body-part (such as the relative blood volume, its velocity, flow, and the like).
The result of the above-described analysis may also be represented graphically by means of a so-called parametric image (or map). The parametric image is built by assigning the respective values of a selected perfusion parameter to each pixel. Typically, different ranges of values of the perfusion parameter are coded with corresponding colors; the pixel values so obtained are then overlaid on one of the original images. In this way, the parametric image shows the spatial distribution of the perfusion parameter throughout the body-part under analysis.
However, although the parametric images may facilitate the identification of possible portions of the body-part that are abnormally perfused, they simply provide a static representation of the perfusion parameters. Therefore, the parametric images do not allow a direct visual perception of the perfusion process, which is normally provided by the playback of the original sequence of images.
In any case, the parametric analysis techniques usually require time-consuming processing of the information that was recorded; therefore, the obtained results are only available off-line (i.e., not in real-time during the perfusion process).